The present invention relates generally to the field of fluid flowmeters which respond to the influences of vortices generated within the fluid stream and more particularly to flowmeters of this type in which the sensing of the responses to the vortex action is performed away from the fluid stream.
It is well known in the art of fluid flowmeter design that a non-streamlined, or bluff, body placed appropriately within a fluid stream sheds a series of periodic vortices, commonly known as a Von Karmann vortex street, alternately from opposite sides of the body, the rate of generation being related to the flow rate of the fluid. Numerous flowmeter apparatus have been devised in which a sensing element, located downstream from the bluff body, interacts in some manner with the vortices as the vortices, propagating with the fluid, move past the element. For example, the sensing element, if positioned properly, will be subject to periodic pressure pulses, induced by the vortices alternately on opposite sides of the element. Commonly used sensing elements are able to convert these alternating pressure pulses to, for example, an electrical signal which is processed and interpreted electronically to yield an accurate representation of the flow rate of the fluid.
There are several drawbacks, however, to placing the sensing element directly in the fluid stream. First of all, exposure to the process fluid, which may be highly abrasive or corrosive or at extremes of temperature, causes deterioration of the sensor and shortens its operating lifetime. Even if the sensor is not destroyed totally by the process fluids, it is very likely that the accuracy of its measurements will be affected adversely as it slowly deteriorates. Second, replacement of a efective sensing element is extremely difficult since this generally necessitates disruption in the flow of the process fluid and disassembly of the pipe section in which the sensor is located, resulting in costly downtime. With this in mind, it is particularly advantageous to position the sensing element outside of the pipe section and away from the destructive effects of the process fluid, in a more environmentally acceptable location. In such an arrangement, it becomes necessary to transmit the mechanical effects induced by the vortices, such as vibration or rotary motion, from the inside of the pipe to the location of the sensor.
Several systems for external transmission of the vortex-induced motion have been proposed in the prior art. Non-mechanical systems utilizing, for example, magnetic coupling, ultrasonic or radioactive techniques have been developed, with varying degrees of success. A commonly used means of transmission is a system in which vibrations or pulsations are induced within a body positioned within the vortex street, and are transmitted, by a mechanical linkage coupled to the body, to the exterior of the pipe. The sensor then interacts with the mechanical linkage to generate an output signal corresponding to the flow rate of the fluid. The mechanical linkage yields a positive transmission of the flow rate information to the external sensor.
Representative examples of such previous designs are found in U.S. Pat. Nos. 3,116,639 (Bird); 3,946,608 (Herzl); and 4,033,189 (Herzl et al). Each of these examples, however, incorporates a feature which adversely affects the ability of the body and its associated mechanical linkage to transmit information signals, induced by the influences of the vortices, beyond the pipe interior. For example, in Bird, a rubber-like gasket material, used as a fluid seal around the body's rotational shaft, unavoidably exerts considerable friction against the shaft, to reduce its freedom of rotation. This situation decreases the sensitivity of the flowmeter, especially when measuring streams having very low flow rates. In the Herzl and the Herzl et al patents there is a restraint on the freedom of movement of a rotatable body due to the fact that a portion of the body is secured to the interior of the pipe wall. In each of these cases there is a dissipation of the kinetic energy initially produced within the rotatable body by the vortices, so that the amount of motion ultimately transmitted to and detected by the external sensor becomes attenuated. Such attenuation means loss of information and loss of ability to measure very low flow rates.
Therefore, it is an object of the present invention to produce a vortex-sensing flowmeter in which a rotatable body influenced by the vortex street experiences a minimal amount of restraint to rotational motion within the interior of the pipe, to insure maximum transmission of informational signals to a sensor located outside of the pipe.
Another object of the invention is to isolate the sensor away from the process environment, to avoid detrimental influences on the sensors, and to facilitate maintenance thereof.
Still another object is to minimize the susceptibility of the flowmeter to respond to vibrations other than those caused by vortex shedding.
Yet another object of the invention is to achieve a flowmeter assembly which exhibits significant sensitivity to fluid flow even at extremely low flow rates, and to do so in a reliable and accurate manner.